The cardiovascular and metabolic adjustments to exercise in cold environments have received minimal attention. It is not known whether thermoregulation is modified during rest or exercise in aging man. Male subjects (18-80 years old) will rest and perform maximal work and three levels of submaximal work on a bicycle in 5 C and 23 C environmental temperatures. Cardiovascular respiratory, metabolic and endocrine responses will be monitored in an attempt to determine if greater stress is placed on older individuals during work in the cold and whether the severity of stress is related to physical fitness, body fatness, lean body mass, aging per se, or other factors. The proposed investigation will determine whether or not thermal balance can be achieved by the elderly during work in cold environments. It will provide valuable insight into mechanisms responsible for age-related differences in cold tolerance and the best possible means for physiologically improving comfort in the cold.